3.0.20 breaks experimental_onFunctionCall in useChat

[MrOrz]

Description

Steps to reproduce

1. Follow quickstart https://sdk.vercel.ai/docs/getting-started
2. Follow "Defining functions" and "Handling Function Calls on the Client" in OpenAI functions doc https://sdk.vercel.ai/docs/guides/providers/openai-functions#handling-function-calls-on-the-client , which implements functionCallHandler on client code to process the function call from LLM.

The repository that can reproduce the issue: https://github.com/MrOrz/vercel-ai-function-bug/

Expected result

This screenshot is made by pinning ai SDK at version 3.0.19:

1. console prints function call argument, indicating that the functionCallHandler is invoked correctly
2. function return value is added to messages and chat API is invoked again, as expected

Actual result

This is the result of 3.0.20 and 3.0.22:

Note that console prints nothing -- functionCallHandler is probably not called at all

Code example

https://github.com/MrOrz/vercel-ai-function-bug/

Additional context

#1316 indicates that there is a breaking change in 3.0.20. But it is more related to custom parsing of StreamingTextResponse. Does not mention that 3.0.20 also breaks experimental_onFunctionCall.

[lgrammel]

I have just tested the openai function call example incl. client side execution ( https://github.com/vercel/ai/tree/main/examples/next-openai/app/function-calling ) and it works as expected. I'll check out your repository to see if I can find out what's different.

[lgrammel]

Workaround: you can use this to create the stream:

  const stream = OpenAIStream(response, {
    async experimental_onFunctionCall() {
      return;
    },
  });

I'll investigate how to automatically support function calls like this.

[johnson-liang]

Thanks, the workaround works for us.
I can help sending PR to amend the documentation if the workaround would stick around for a while.

[lgrammel]

@johnson-liang thanks. No need for a PR, we're in the process of updating our docs, so your changes would get lost.

[lgrammel]

With the recently added useChat / streamText tool calling revamp (see https://sdk.vercel.ai/docs/ai-sdk-ui/chatbot-with-tool-calling ), experimental_onFunctionCall has become deprecated.